The present invention relates broadly to filters for chemical-mechanical polishing (CMP) slurries, and more particularly to a woven fabric filter element therefor which mimics the sieve-like performance of more expensive track-etched membranes but with an improved service life.
In the general mass production of semiconductor devices, hundreds of identical "integrated" circuit traces are photolithographically imaged over several layers on a single semiconducting wafer which, in turn, is cut into hundreds of identical dies or chips. Within each of the die layers, the circuit traces are insulated from the next layer by an insulating material. Inasmuch as it is difficult to photolithographically image a rough surface, it is desirable that the insulating layers are provided as having a smooth surface topography or, as is termed in the vernacular, a high degree of planarity. In this regard, a relatively rough surface topography may result in poor coverage by subsequently deposited layers, and in the formation of voids between layers. As circuit densities in semiconductor dies continue to increase, any such defects become unacceptable and may render the circuit either inoperable or lower its performance to less than optimal.
To achieve the relatively high degree of planarity required for the production of substantially defect free dies, a chemical-mechanical polishing (CMP) process is becoming increasingly popular. Such process involves chemically etching the wafer surface in combination with mechanical polishing or grinding. This combined chemical and mechanical action allows for the controlled removal of material.
In essential operation, CMP is accomplished by holding the semiconductor wafer against a rotating polishing surface, or otherwise moving the wafer relative to the polishing surface, under controlled conditions of temperature, pressure, and chemical composition. The polishing surface, which may be a planar pad formed of a relatively soft and porous material such as a blown polyurethane, is wetted with a chemically reactive and abrasive aqueous slurry. The aqueous slurry, which may be either acidic or basic, typically includes abrasive particles, a reactive chemical agent such as a transition metal chelated salt or an oxidizer, and adjuvants such as solvents, buffers, and passivating agents. Within the slurry, the salt or other agent provides the chemical etching action, with the abrasive particles, in cooperation with the polishing pad, providing the mechanical polishing action. The basic CMP process is further described in the following U.S. Pat. Nos.: 5,709,593; 5,707,274; 5,705,435; 5,700,383; 5,665,201; 5,658,185; 5,655,954; 5,650,039; 5,645,682; 5,643,406; 5,643,053; 5,637,185; 5,618,227; 5,607,718; 5,607,341; 5,597,443; 5,407,526; 5,395,801; 5,314,843; 5,232,875; and 5,084,071.
Looking to FIG. 1, a representative CMP process and apparatus therefor are illustrated schematically at 10. The apparatus 10 includes a wafer carrier, 12, for holding a semiconductor wafer or other workpiece, 14. A soft, resilient pad, 16, is positioned between wafer carrier 12 and wafer 14, with the wafer being held against the pad by a partial vacuum, frictionally, or with an adhesive. Wafer carrier 12 is provided to be continuously rotated by a drive motor, 18, in the direction referenced at 20, and additionally may be reciprocated transversely in the directions referenced at 22. In this regard, the combined rotational and transverse movements of the wafer 14 are intended to reduce the variability in the material removal rate across the work surface 23 of the wafer 14.
Apparatus 10 additionally includes a platen, 24, which is rotated in the direction referenced at 26, and on which is mounted a polishing pad, 28. As compared to wafer 14, platen 24 is provided as having a relatively large surface area to accommodate the translational movement of the wafer on the carrier 12 across the surface of the polishing pad 28.
A supply tube, 30, is mounted above platen 26 to deliver a stream of polishing slurry, referenced at 32, which is dripped or otherwise metered onto the surface of pad 28 from a nozzle or other outlet, 34, of the tube 30. The slurry 32 may be gravity fed from a tank or reservoir (not shown), or otherwise pumped through supply tube 30. Alternatively, slurry 32 may be supplied from below platen 26 such that it flows upwardly through the underside of polishing pad 28.
Slurries for CMP, which are further described in U.S. Pat. Nos. 5,516,346; 5,318,927; 5,264,010; 5,209,816; 4,954,142, may be of either an oxide, i.e., ceramic, or metal abrasive particle type. Common oxide-type particles include silica (SiO.sub.2), ceria (CeO.sub.2), silicon carbide (SiC), silicon nitride (Si.sub.3 N.sub.4), iron oxide (Fe.sub.2 O.sub.3), alumina (Al.sub.2 O.sub.3), and the like, with common metal particles including tungsten and copper. The slurry typically is formulated to have a very high solids level of between about 10-12% by weight, with a mean average abrasive particle size of between about 0.05-0.3 .mu.m for oxide slurries and about 20-35 .mu.m for tungsten slurries.
It has been observed, however, that as a result of agglomeration and drying from exposure to air, larger particles of a particle size of about 50.0 .mu.m or more may develop within the slurry. Although the metal-type slurries generally are more susceptible to agglomeration than the oxide types, the problem may present in either type of slurry depending upon the slurry composition and ambient conditions. Should the agglomerated particles be entrained within the CMP slurry, significant damage to the to the wafer surface being planarized can result. Moreover, it is known that to achieve a low defect rate and high wafer yield, each successive wafer substrate should be polished under substantially similar conditions.
It therefore has been proposed to filter the CMP process stream at the point of use to separate agglomerated particles of a size larger than a predetermined limit from the balance of the slurry. Initially, filters employing conventional membranes elements, which may be of a phase inversion or bi-axially stretched variety generally having particle retention ratings between about 0.3-0.65 .mu.m, were suggested. In service, however, membranes filters of such type were observed to load almost instantaneously with particulate and soon were judged unacceptable for the CMP process. The characteristics of conventional membrane filter media are described in greater detail in U.S. Pat. Nos. 5,449,917; 4,863,604; 4,795,559; 4,791,144; 4,770,785; 4,728,394; and 3,852,134.
Alternative filter elements which have met with more success in the CMP process employ fibrous media such as randomly orientated webs. Indeed, unlike membranes which rely on surface-type filtration, these fibrous media utilize a tortuous path, depth-type filtration mechanism. In order to provide acceptable service life, however, a fibrous media must be selected as having a relatively open and permeable structure rated, for example, at about 40-100 .mu.m absolute or 5-30 .mu.m nominal. Such a rating ensures substantially no retention of particles in the 0.5-2 .mu.m range which could cause cake formation and, ultimately, premature blockage of the filter element. As a drawback, the more open and permeable structure does allow for some passage of large size particles which could damage the substrate being planarized. That is, fibrous media in general characteristically exhibit a gradually decreasing retention profile as a function of decreasing particle size which is in contrast to the sharper particle size cutoff exhibited by membranes and other surface-type media. Depth-type and other filter media are described in further in U.S. Pat. Nos. 5,637,271; 5,225,014; 5,130,134; 4,225,642; and 4,025,679.
In view of the foregoing, it will be appreciated that further improvements in the design of elements for CMP process would be well-received by the semiconductor manufacturing industry. Especially desired would be a filter element exhibiting a particle retention profile which is comparable to surface filtering membranes, but with a service life which is more like that of a depth filtering media.